The Physiological Bases Of The Unsuitable Host Insect Restricting The Success Of Cotesia Vestalis Development

In this study,four kinds of insects,such as Cotesia vestalis,Diadegma semiclausum,Plutella xylostella and Helicoverpa armigera were used.Three host-parasitoid systems were established: There are Plutella xylostella-Cotesia vestalis system,Plutella xylostella-Diadegma semiclausum system and Helicoverpa armigera-Cotesia vestalis system,respectively.We mainly focused on the research of the physiological bases of the unsuitable host insect restricting the success of Cotesia vestalis development.This paper included four parts:The embryonic development of Cotesia vestalis or Diadegma semiclausum in their host larvae,Plutella xylostella;The effects of host instars and ages on the development of the teratocytes and larvae of Cotesia vestalis;The effects of parasitism on the development of Plutella xylostella larvae induced by two endoparasitoids,Cotesia vestalis or Diadegma semiclausum,respectively;Study on the unsuitable host restricting the successful development of Cotesia vestalis.In this study,Plutella xylostella was regarded as a suitable host to Cotesia vestalis and Diadegma semiclausum,whereas Helicoverpa armigera was regarded as an unsuitable host to Cotesia vestalis.The results were shown below:1.Embryonic development of two endoparasitoids in their host larvae,Plutella xylostellaThe embryonic development of two larval endoparasitoids of Plutella xylostella was observed by dissecting parasitized hosts in the laboratory at(25±1)℃.The development period required for Cotesia vestalis embryo was about 48 h.It passes through mainly three rather different stages according to their developmental succession.The first stage included the cleavage and blastoderm stages,cleavage ends with the formation of a blastula after 20 h oviposition.The second phase was called gastrulation and organic development stages from 20 h to 32 h after oviposition,including germ band formation,germ band expanding,differentiation of protocorn and protocephalon,formation of the alimentary canal and mouthparts,and organic formation.The last phase included the dorsal closure and embryo maturation stages from 32 h to 48 h after oviposition.Obviously,a lots of teratocytes originated from the serosal membrane were released into the host larval haemocoel.At the same condition,the development of the embryo of Diadegma semiclausum was about 50 h.The process of the embryonic development was also divided into three stages on the basis of the characterization of embryo morphology.Just like C. vestalis.After 12 h oviposition,a blastula was formed.And then starting the gastrulation and organic development stages from 12 h to 32 h after oviposition.In the end,accompanied with the dorsal closure,the embryo began to mature.Fifty-hour later,the first larva of D.semiclausum hatched,but very different from C.vestalis,its serosal membrane could not produce tetatocytes at all.2.The effects of host instars and ages on the development of the teratocytes and larvae of Cotesia vestalisDifferent host instars have different suitability to the development of the koinobiont endoparasitoids.Choosing the suitable host instar and size will be useful for developing augmenting parasitoids as biological control agents.In Plutella xylostella-Cotesia vestalis system, the effects of different host instars on the development of teratocyte and larvae of C.vestalis were different.The results showed that tartocyte numbers decreased and size increased as parasitoid development progressed.We determined that the 3rd host instar was the most suitable host to the development of C.vestalis both in parasitoid larvae and teratocyte.Firstly,in the development of parasitoid larvae,we compared the development of parasitoid larvae in different host instars of P. xylostella larvae,only in 3rd instar the parasitoid larvae developed quickly and regularly. Secondly,in the development of teratocyte,the 3rd instar of P.xylostella larvae supported more C. vestalis-derived teratocytes than other host instars,and most of the teratocytes developed regularly. However,in other host instars,the teratocytes grew larger or smaller than in 3rd insatr.We concluded that teratocytes growth pattern reflected host instar suitability.3.The effects of parasitism on the development of Plutella xylostella larvae induced by two endoparasitoids,Cotesia vestalis or Diadegma semiclausum,respectivelyWe examined growth and development of the 3rd Plutella xylostella larvae parasitized by two endoparasitoids,Cotesia vestalis or Diadegma semiclausum,respectively.In this study,during the six days post parasitization,we examined five nutrition index about P.xylostella:Relative growth rate(RGR),Relative consumption rate(RCR),Eficiency of conversion of ingested food(ECI),Eficiency of conversion of digested food(ECD) and Approximate digestibility(AD).The results showed that the relative growth rate of parasitized P.xylostella larvae was obvirously lower than that of unparasitized host larvae.Also,there was difference in the relative growth rate between the host parasitized by two endoparasitoid,respectively.In different treated host larvae,besides RGR other nutrition index including RCR,ECI,ECP and AD were affected by parasitism.The results showed that the feeding capacity of parasitized P.xylostella larvae increased significantly,whereas the host larval body weight increased slowly.The rate of nutritional utilization utilizeded by parasitized host was lower than that of control.It suggested that the nutrition was transferred to parasitoid larva which lived in the haemolymph of the parasitized host larva.Finally,an advanced environment in phsiology and nutrition for the development of larval parasitoid was formed.4.The physiological bases of unsuitable host restricting the success of Cotesia vestalis developmentFor endoparsitoids,host suitiblity directly influence the results of insect enemy introduction in biological control.To determine the factors which can be used useful criterior to evaluate host suitability and host range of parasitoids,three kinds of insects,such as Cotesia vestalis,Helicoverpa armigera and Plutella xylostella were chosen.In different fitness hosts,we studied on the embryonic development of C.vestalis,larval development of C.vestalis,the development of teratocytes,and the developmental period of its egg-cocoon,cocoon-eclosion,and their adult life time.At the same time,we measured the length of forewing and hind tibia,the parasitism rate,the successful parasitism rate and the rate of cocoon formation.The results showed that in suitable host Plutella xylostella,the embryonic development of C.vestalis was rapider and healthier than that in unsuitable host Helicoverpa armigera.In H.armigera,the parasitoid larval volume was obviously different from that in P.xylostella.Seven days after oviposition,the volume of larvae in P.xylostella was bigger than that in the unfiness hosts H.armigera.Furthermore,most of parasitoid larvae developed abnormally or melanized in the unfitness hosts of H.armigera.A delay in the parasitoid larval development was observed.However,9 days after oviposition the volume of larvae in the unfitness hosts continued to increase,at last,they developed bigger than normal parasitoid larvae which developed in P.xylostella.The teratocytes development pattern in differernt suitability hosts was obviously different.The original number of teratocytes released into H.armigera haemolymph was less than that in P.xylostella,and the number of teratocytes in H.armigera decreased more quickly than that in P.xylostella.In the unfitness hosts H.armigera, the diameter of teratocytes increased little after 5 days oviposition,but it could increase rapidly in the last time.The biggest diameter of teratocytes in H.armigera was almost 3 times bigger than that in P.xylostella.The morphology of teratocytes changed diversily in H.armigera.Some teratocytes were adherred by host haemocytes,and some of them melanized.The developmental period of C.vestalis was longer than that in P.xylostella,and the cocoon size of C.vestalis formed from the unfitness host H.armigera was bigger than that from the fitness host P.xylostella, eventually the adult size of C.vestalis had the same trend.The adult life time of C.vestalis which developed from H.armigera was longer than that from P.xylostella.All in all,Teratocyte growth and development pattern can be used a useful criterion to evaluate the host suitability to endoparsitoid.